In internal combustion engines, nickel base alloys with high carbon and high chromium content (i.e., about 20 wt. % or greater) have been widely used as exhaust valve seat insert materials because of their good wear resistance and excellent oxidation resistance as well as excellent hot hardness properties. The microstructures of these nickel base alloys can be characterized as high volume fraction of massive M.sub.7 C.sub.3 and M.sub.23 C.sub.6 type carbides embedded in a nickel rich solid solution matrix, strengthened by solute atoms like chromium, tungsten or molybdenum elements. Often these alloys require a high percentage of expensive nickel element (i.e., about 45 wt. % or greater) and even a certain amount of cobalt in some alloys, contributing to the high cost of manufacturing these alloys.
Exhaust valve seat inserts made from these alloys generally provide satisfactory service life in the current diesel fuel engines. However, as emission standards tend to become tighter, less combustion deposits are available as media between valve and insert seating surfaces that, in the past, have served as a protective coating to reduce direct metal-to-metal contact between the valve and valve seat members. As such deposits become less available, the traditional nickel base alloys are prone to undesirable metallic sliding wear due to such metal-to-metal contact due to their microstructures and chemical compositions of the matrix, and thus direct metal-to-metal contact of valve and valve seat insert surfaces leads to premature wear of valve seat inserts. Moreover, ever increasing demand on engines for more power output per unit cylinder volume increases the load and worsens the working conditions of these nickel base alloys.
Many of the traditional nickel base alloys originated from materials used in valve hardfacing applications, where hot hardness and oxidation resistance are of special importance to these alloys because of much higher temperature (i.e., about 1200.degree. F.) at the working surfaces of valves. However, valve seat inserts per se are not subjected to such high working temperatures (i.e., typically only around 800.degree. F.), and as such the materials they are made from do not need to exhibit the same hot hardness and oxidation resistance properties, because of their lower working temperature. Hence there is a need in the industry for a nickel base alloy with improved sliding wear resistance and with other engineering properties which are specifically adapted for valve seat insert applications.
An essential feature of many prior art nickel base alloys is that high chromium content is required to obtain maximum corrosion resistance or to form acicular chromium carbide for better abrasion resistance, as disclosed, for example, in U.S. Pat. Nos. 4,075,999, 4,191,562, 4,228,223, 4,430,297, 5,246,661, 5,360,592, where chromium ranges between 20.0 to 30.0 wt. % or higher. Several well known commercial valve seat insert alloys, as shown in Table 1 below, belong to this group because of their high chromium content. Some of these nickel base alloys at the same time control iron content to a minimum or low level, as described, for example, in U.S. Pat. Nos. 4,075,999, 4,191,562, 4,228,223 and 4,279,645. For better high temperature properties, a certain amount of cobalt is added to some nickel base alloys, as shown, for example, in U.S. Pat. Nos. 4,191,562 and 4,279,645.
U.S. Pat. No. 4,810,464 discloses an iron base alloy with 27.0 to 43.0 wt. % nickel, 0.1 to 5.0 wt. % silicon, up to 10.0 wt. % chromium, 0.2 to 1.5 wt. % carbon, 3.0 to 5.0 wt. % boron. Noticeably, refractory elements such as molybdenum and tungsten are absent in the alloy, indicating the alloy is intended for moderate temperature applications.
Another known wear resistant nickel base alloy is a composition containing 0.3-2.0 wt. % C, 15.0-25.0 wt. % Cr, 2.0-5.0 wt. % Mo, 1.0-12.0 wt. % Fe, 5.0-20.0 wt. % Co, 0.5-2.0 wt. % Al, as disclosed in U.S. Pat. No. 4,279,645, where high tensile strength at elevated temperatures is the primarily objective for aircraft gas turbine applications.
A nickel-iron base alloy (U.S. Pat. No. 4,292,074), used for rocker arm pads in overhead camshaft combustion engines contains essentially 0.5-2.0 wt. % C, 6-1.0 wt. % Si, 0.5-3.0 wt. % B, 30.0-60.0 wt. % Fe, 30.0-60.0 wt. % Ni, and the total amount of Cr, Mo, and W is 2.0-8.0 wt. %. As disclosed in the patent, maximum wear resistance is obtained when silicon content is in the 6.0 to 10.0 wt. % range under lubricated condition.
TABLE 1 Some Prior Art Valve Seat Insert Alloy Compositions (wt. %) Alloy Name C Si Cr Mo W Co Fe Ni W230 (Eatonite 8) 1.8-2.5 1.5 Max. 27.0-31.0 7.0-9.0 1.0 Max. -- 25.0 Max. Bal. W240 (Eatonite 2) 2.0-2.75 1.0 Max. 27.0-31.0 -- 14.0-16.0 -- 8.0 Max. Bal. W250 2.25-2.75 .4-1.1 26.5-30.5 1.5-2.5 1.5-2.5 15.0-18.0 9.0 Max. Bal. W260 (Eatonite) 2.0-2.75 1.0 Max. 27.0-31.0 -- 14.0-16.0 9.0-11.0 8.0 Max. Bal. W280 (Super Eatonite) 1.3-1.5 1.0 Max. 25.0-28.0 9.0-11.0 9.0-11.0 9.0-11.0 10.5-14.0 Bal.